EP 1 107 441 A2 and corresponding U.S. Pat. No. 6,396,226, SCHMIDER & LUKENICH, disclose an ECM in which an arrangement for current limiting is provided. Current limiting is important in particular upon startup of a motor, since when a motor's rotation speed is low, the voltage induced in its stator winding by the permanent-magnet rotor—the so-called counter-EMF—is also low; at startup, this can result in high currents which are substantially greater than the rated current of such a motor. If no current limiter is provided, the magnitude of the current spikes upon startup is limited only by the winding resistance and by the magnitude of the counter-EMF (which is low at low rotation speeds). These high currents result in corresponding motor noise, which is present in the audible range and whose frequency corresponds to the commutation frequency. This is explained below with reference to FIG. 3.
It is known from EP 1 107 441 A2 and U.S. Pat. No. 6,396,226 to suppress current limiting at startup in order to permit fast acceleration. This is done by connecting a bypass transistor in parallel with a current measuring resistor. This bypass transistor is made conductive for 0.5 second at startup, and during that period no current limiting occurs. This elevation of startup current is permissible only with small motors, however, and requires a high-power bypass transistor that must be capable of conducting the greatly elevated starting current for 0.5 second.